The influences of the mass ratio of slag to desulfurization gypsum, cement, alkaline activator quicklime powder, water-binder ratio and water-reducing agent on the mechanical properties of the undisturbed desulfurization gypsum-based hydraulic cementitious material system were studied systematically to determine the optimum mix proportion. The effects of the digestion time and the curing conditions on the mechanical properties of the cementitious material system were also investigated to determine the optimum preparation process. The results show that the optimum mass ratio of slag to desulfurization gypsum is 4∶6.The optimum mass fractions of cement, quicklime and water-reducing agent are 8%, 7% and 0.7%, respectively. The optimum water-binder ratio is 0.26. These components are mixed by the mass ratio on the basis of the overall mass of gypsum and slag. The optimum digestion time of the undisturbed desulfurization gypsum hydraulic cementitious material system is 18 h. The optimum steam curing age is 21 h and the optimum steam temperature is 60 ℃.